[0001] The invention relates to a process for the preparation of urea from ammonia and carbon
dioxide.
[0002] When ammonia and carbon dioxide are introduced into a synthesis zone at a suitable
pressure (for instance 125-350 atm) and at a suitable temperature (for instance 170-250◊C),
first ammonium carbamate is formed according to the reaction:
2 NH₃ + CO₂
H₂N-CO-ONH₄
[0003] From the ammonium carbamate formed, urea is subsequently formed through dehydration
according to the reversible reaction:
H₂N-CO-ONH₄
H₂N-CO-NH₂ + H₂O
[0004] The degree to which the conversion to urea takes place depend, inter alia, on the
temperature and the ammonia excess used. As reaction product, a solution is obtained
that consists mainly of urea, water, ammonium carbamate and free ammonia. The ammonium
carbamate and the ammonia are to be removed from the solution; mostly, they are returned
to the synsthesis zone. This synthesis zone may consist of separate zones for carbamate
and urea formation, but these zones may also be accommodated in one apparatus.
[0005] One process for the preparation of urea that has found wide use in practical applications
is described in European Chemical News, Urea Supplement of January 17, 1969, pages
17-20.
In said process, the urea synthesis solution formed at high temperature and pressure
in the synthesis zone is subjected to a stripping treatment at synthesis pressure
by countercurrently contacting the solution with gaseous carbon dioxide while supplying
heat, so that the larger part of the carbamate present in the solution decomposes
into ammonia and carbon dioxide, and these decomposition products are in gaseous form
expelled from the solution, and discharged together with a minor amount of water vapour
and the carbon dioxide used for stripping. The heat required for the stripping treatment
is obtained by condensation of high-pressure steam of 15-25 bar, on the shell side
of the tubes of the vertical heat exchanger in which stripping takes place.
[0006] The gas mixture obtained in the stripping treatment passes to a first condensation
zone and is for the larger part condensed and absorbed in an aqueous solution originating
from the further treatment of the urea-containing solution, upon which both the aqueous
carbamate solution thus formed and the non-condensed gas mixture are sent to the
synthesis zone for urea formation. Here, the heat required for the conversion of carbamate
into urea is obtained by further condensation of the gas mixture.
[0007] The stripped urea synthesis solution is subsequently expanded to a low pressure of,
for instance, 3-6 bar and heated by means of steam so as to remove the ammonia and
carbon dioxide still partly present as carbamate from the stripped urea solution.
The gas mixture obtained in these operations, which also contains water vapour, is
condensed and absorbed in an aqueous solution in a second condensation zone, which
is operated at low pressure, and the resulting dilute carbamate solution is returned
to the high-pressure section of the urea synthesis and eventually introduced into
the synthesis zone. The remaining urea-containing solution is reduced further in pressure
and is worked up to a urea solution or melt that may be processed to solid urea. To
this end, the aqueous urea solution is usually evaporated in two evaporation stages
and the urea melt thus obtained is processed to granules, or the urea solution is
crystallized. The gases obtained during evaporation or crystallization, which besides
water vapour contain, inter alia, ammonia, carbon dioxide and entrained fine urea
droplets, are condensed, yielding so-called process condensate. A portion of the
process condensate is used as absorption agent for the gas mixture in the second condensation
zone. The remainder can be treated with high-pressure steam for decomposition into
ammonia and carbon dioxide of urea contained in it and recovery of these decomposition
products together with the ammonia and carbon dioxide already present as such.
[0008] It has already been proposed to incorporate an additional decomposition stage in
such a process in which further amounts of carbamate, that are still present in the
stripped urea synthesis solution, are decomposed at a pressure of 12-25 kg/cm² (see
US patent 4,354,040). A drawback of such an additional decomposition stage is that
the molar ratio of the ammonia and carbon dioxide not converted into urea in the urea-containing
solution discharged from this additional decomposition stage is relatively high.
As a consequence, this ratio will also be relatively high in the gas mixture formed
on decomposition of carbamate still present in the further decomposition stages.
[0009] For complete condensation into a crystal-free carbamate solution of such gas mixtures,
considerable amounts of water or water-containing absorbents are required. If, as
is done in the process described, a portion of the fresh carbon dioxide required in
the synthesis is supplied to the last decomposition stage, the gas mixture obtained
in this stage is given an NH₃/CO₂ molar ratio that is more favourable for complete
condensation under the prevailing conditions. In this condensation, however, the heat
of condensation is released at the low temperature level belonging to this decomposition
stage, as a consequence of which there are hardly any possibilities for efficient
use of the heat released, necessitating its discharge by means of cooling water. Another
drawback is that the carbon dioxide supplied to the last decomposition stage is no
longer available for the stripping treatment in the high-pressure part, so that a
larger amount of carbamate remains in the stripped urea synthesis solution, which
must be removed in the following decomposition stages.
[0010] The object of the invention is to provide a process for the preparation of urea which
avoids the above-mentioned drawbacks. According to the invention this can be achieved
if only a portion of the urea synthesis solution is treated in an additional decomposition
stage at 12-30 bar, without prior heat supply. It has been found that, if subsequently
further amounts of carbamate still present, both in the portion treated in the additional
decomposition stage and in the portion of the solution that originates from the stripping
treatment, are decomposed at lower pressures than the pressure in the additional decomposition
stage, without supply of fresh carbon dioxide in this stage a gas mixture is obtained
the NH₃/CO₂ ratio of which is such that the gas mixture can be condensed without addition
of excessively large amounts of water.
[0011] The invention therefore relates to a process for the preparation of urea in which:
- a urea synthesis solution containing carbamate and free ammonia is formed in a high-pressure
part in a synthesis zone at an NH₃/CO₂ molar ratio of up to 4 : 1, a temperature of
at least 175◊C and the corresponding pressure,
- a portion of the carbamate is decomposed in a first decomposition stage at synthesis
pressure or lower pressure by a stripping treatment with carbon dioxide while heat
is being supplied, and the gas mixture thus obtained is at least in part condensed
and the condensate and the non-condensed portion of the gas mixture, if any, are returned
to the synthesis zone,
- a further portion of the carbamate still present is decomposed in at least two further
decomposition stages and the gas mixture formed is separated, in the first of the
further decomposition stages a pressure of 12-30 bar being maintained and heat being
supplied and in the second of the further decomposition stages a lower pressure being
maintained,
- and the remaining urea-containing solution is processed further by evaporation to
a concentrated urea solution and, if desired, solid urea.
[0012] The process according to the invention is characterized in that in the first decomposition
stage a portion of the urea synthesis solution is subjected to a stripping treatment
with carbon dioxide while heat is being supplied, and the remaining portion of the
urea synthesis solution is countercurrently contacted with carbon dioxide under adiabatic
conditions, after which the gas mixtures obtained in both operations are at least
in part condensed in a first condensation zone, the solution obtained in the treatment
of the urea synthesis solution with carbon dioxide under adiabatic conditions is supplied
to the first of the further decomposition stages and the stripped urea synthesis solution
to the second of the further decomposition stages.
[0013] Preferably. 50-70 wt.% of the urea synthesis solution is subjected to the stripping
treatment with carbon dioxide while heat is being supplied, and 50-30 wt.% to the
treatment with carbon dioxide under adiabatic conditions. The amount of carbon dioxide
required in the urea process is divided between both treatment stages in virtually
the same proportion.
[0014] As a result of the treatment with carbon dioxide under adiabatic conditions of a
portion of the urea synthesis solution, the latter's ammonia content is reduced and
the carbon dioxide content increased. A portion of the carbamate still present in
it is decomposed by heating in the first of the further decomposition stages at a
pressure of 12-30 bar, after the gas mixture formed on expansion to a pressure of
12-30 bar has been separated. Heating can be effected by means of, for instance, steam.
By preference, heating is effected by heat exchange with the condensing gas mixture
at synthesis pressure in the first condensation zone. If the conditions in the first
condensation zone are chosen such that also a considerable amount of urea, for instance
at least 30 % of the equilibrium amount achievable under the reaction conditions,
is formed from the carbamate formed in condensation, the heat will be released at
such a temperature level that a considerable portion of the carbamate present in the
solution can be decomposed into ammonia and carbon dioxide. As a rule, there will
even be a surplus of heat. This heat can then be discharged in a second high-pressure
condensation zone by means of boiler feed water, which is thereby converted into low-pressure
steam of 4-9 bar. The gas mixture formed on carbamate decomposition as a result of
heat exchange with the condensing gas mixture in the first condensation zone, can,
for instance, be condensed together with the gas mixture obtained on expansion of
the solution that has undergone the treatment with carbon dioxide under adiabatic
conditions. As a result of the treatment of a portion of the urea synthesis solution
with carbon dioxide under adiabatic conditions, the NH₃/CO₂ molar ratio in the gas
mixture obtained in the first of the further decomposition stages in the 12-30 bar
pressure range will be close to the value for the azeotropic condition, so that in
condensation of this gas mixture the maximum attainable temperature is virtually reached.
[0015] By preference, this condensation is effected using the carbamate solution obtained
on further processing of the stripped urea synthesis solution in a decomposition stage
operating at a pressure of, for instance, 1-10 bar, which carbamate solution is first
brought up to the prevailing pressure in the 12-30 bar pressure range by means of
a pump. The heat of condensation can then be obtained at a level of 145-110◊C, which
is substantially higher than in the process according to the above-mentioned US patent
4,354,040. The heat released in condensation can, for instance, be utilized by heat
exchange with the urea solution to be evaporated. When this is effected by passing
the urea solution to be evaporated countercurrent to the condensing gas mixture, the
urea solution can, for instance, be concentrated at temperatures between 85 and 130◊C
from a urea concentration of about 70 wt.% to about 95 wt.%. These values largely
correspond with the concentration normally achieved in the first evaporation stage
in the above-mentioned process known from European Chemical News.
[0016] The solution remaining after separation of the gas mixture released in the first
of the further decomposition stages, which has a relatively high NH₃/CO₂ molar ratio,
is expanded to the pressure of the second of the further decomposition stages, for
instance to a pressure of 1-10 bar. The solution that remains after the stripping
treatment with heat supply and that has a relatively low NH₃/CO₂ ratio is also expanded
to the pressure of the second of the further decomposition stages. If these solution
are jointly subjected to a treatment for decomposition of further amounts of carbamate
still present, for instance to heating by means of low-pressure steam, a gas mixture
is obtained the NH₃/CO₂ molar ratio of which is such as to allow condensation without
supply of excessively large amounts of water.
[0017] Compared with the known process, the process according to the invention has the advantage
that only a relatively small amount of water is required for condensation of the gas
mixtures obtained in the several decomposition stages, which has a favourable effect
on the synthesis efficiency. Since no carbon dioxide is supplied to the low-pressure
stage, the total amount of carbon dioxide required in the synthesis can be supplied
to the stripping treatment and to the adiabatic treatment, so that the heat of condensation
of this carbon dioxide can be put to efficient use, resulting in optimum efficiency
of these treatments. For decomposition of the further amounts of carbamate and expulsion
of the gas mixtures formed thereby in the decomposition stage operating at 12-30
bar, no additional amount of high-pressure steam is needed, as in the known process,
but use is made of the heat content of the gas mixture obtained in the stripping treatment
for this carbamate decomposition. Moreover, the heat content of the gas mixture from
the decomposition stage operating at 12-30 bar can be utilized at a high temperature
level in concentrating of the urea solution obtained to an approx. 95 wt.% solution
by evaporation.
[0018] The invention will be elucidated with reference to the figure without, however, being
restricted thereto.
[0019] In the figure, a synthesis zone is indicated by 1, a stripping zone by 2, a first
and a second high-pressure condensation zone by 3 and 4, respectively, and a scrubbing
zone by 5. Shown as 6 is a contact zone for contacting urea synthesis solution with
carbon dioxide. 7, 8 and 9 are devices for separating liquids and gases. 10 stands
for a heat exchanger and 11 for a carbamate condensation zone operated at low pressure.
The heating zone of the first and that of the second concentration stage are represented
by 12 and 13, respectively, and the associated devices for separation of the water
vapour formed in concentrating by 14 and 15, respectively. A carbamate pump is shown
as 16, while 17, 18, 19 and 20 are expansion valves. Of the urea synthesis solution
formed in urea synthesis zone 1 at a pressure of 125-250 bar, a temperature of 175-220◊C
and an NH₃/CO₂ molar ratio of 2.7-4.0, for instance 139 bar, 183◊C and an NH₃/CO₂
molar ratio of 3.2, which, besides urea and water, contains free ammonia and non-converted
ammonium carbamate, an amount of 30-40 wt.%, for instance 44 wt.%, is supplied via
22 to contact zone 6, where it is contacted with carbon dioxide under adiabatic conditions.
The remaining 50-70 wt.% of the urea synthesis solution is supplied to stripping zone
2, which is placed parallel with contact zone 6, through 23. Countercurrent to the
urea synthesis solution, via 24 carbon dioxide, which has been compressed to synthesis
pressure in a compression device not shown and to which, if desired, passivation air
has been added, is supplied to stripping zone 2 and contact zone 6. Stripping zone
2 is preferably designed as a vertical shell-and-tube heat exchanger. The heat required
in the stripping treatment is obtained by condensation of high-pressure steam of,
for instance, 14-40 bar, in the shell side of the heat exchanger. To promote a good
contact between the urea synthesis solution and the carbon dioxide gas, in contact
zone 6 plates or packing materials can be installed. Likewise, the contact between
gas and liquid can be effected in a falling liquid film. The gas mixture expelled
from stripping zone 2, which besides ammonia and carbon dioxide contains equilibrium
amounts of water vapour, is discharged via 25 with the carbon dioxide used for the
stripping treatment. From contact zone 6, via 26 a gas mixture is discharged which
contains the ammonia expelled from the urea synthesis solution, a portion of the carbon
dioxide supplied and a small amount of water. The gas mixtures discharged via 25
and 26 are passed via 27 into the shell side of first condensation zone 3, represented
in the figure as a submerged condenser, horizontally placed, where they are partially
condensed to a carbamate solution. Via 28, first condensation zone 3 is fed with a
dilute carbamate solution, which is obtained in scrubbing ammonia and carbon dioxide
out of the gas mixture containing inert gases that has been discharged from synthesis
zone 1 via 29. The heat evolved in the formation of this dilute carbamate solution
is utilized for preheating of the liquid ammonia supplied via 30. To this end, a
heat exchanger 21 can be installed, in which the heat released in scrubbing zone 5
is transferred via a circuit 21a. The residence time of the reaction mixture in first
condensation zone 3 is chosen such that in this zone also at least 30 % of the equilibrium
amount of urea achievable under the prevailing reaction conditions is formed from
carbamate, so that a solution with, for instance, 20 wt. % urea is obtained. The heat
released in first condensation zone 3 can be utilized for decomposition of carbamate
present in the urea synthesis solution enriched with carbon dioxide. To this end,
the solution discharged from contact zone 6 via 31 is expanded to a pressure of 12-30
bar, for instance 22 bar, in expansion valve 17, and the mixture formed is introduced
into gas-liquid separator 7. The liquid phase thus formed, mainly an aqueous urea
solution also containing biuret, ammonia and carbamate, is discharged via 32, and
the gas phase, a mixture containing mainly ammonia, carbon dioxide and water vapour,
via 33. The liquid phase discharged from gas-liquid separator 7 via 32 is subsequently
passed, at a pressure equal to or lower than the pressure of gas-liquid separator
7, into heat exchange with the carbamate-urea solution being formed in first condensation
zone 3, upon which a portion of the amount of carbamate present in the expanded solution
is decomposed into ammonia and carbon dioxide. It is also possible to discharge the
heat released in first condensation zone 3 by means of other process streams or with
water, which is thereby converted into low-pressure steam. From first condensation
zone 3, via 34 the non-condensed portion of the gas mixture fed to this zone, and
via 35 the carbamate solution formed in this zone, is discharged and introduced into
second high-pressure condensation zone 4. In this zone, further condensation to a
carbamate solution of the gas mixture supplied via 34 takes place. The heat released
thereby is discharged by means of water, which is thereby converted into low-pressure
steam of 4-9 bar. The carbamate solution obtained in this second high-pressure carbamate
condensation zone 4, and the non-condensed portion, if any, of the supplied gas mixture
containing ammonia, carbon dioxide and water vapour, are passed into synthesis zone
1 via 36.
[0020] The gas liquid mixture obtained in the heat exchange in first high-pressure condensation
zone 3 is passed via 37 into gas-liquid separator 8, from which the gas phase formed,
a gas mixture containing ammonia, carbon dioxide and water vapour, is discharged via
38 and the liquid phase formed, a carbamate-containing urea solution, via 39.
[0021] In expansion valve 18, the liquid phase is reduced in pressure to the pressure of
the second of the further decomposition stage, which normally is in the 1-10 bar range,
for instance 5 bar, and the gas-liquid mixture formed during expansion is fed to gas-liquid
separator 9. The stripped urea synthesis solution discharged from stripping zone 2
is also reduced in pressure to the pressure of the second of the further decomposition
stages, for instance 5 bar, in expansion valve 19, and the gas-liquid mixture formed
is fed to gas-liquid separator 9 via 40.
[0022] Via 41, a urea-containing solution is discharged from gas-liquid separator 9, carbamate
still present in this solution is decomposed in heat exchanger 10, which is heated
by low-pressure steam, after which the urea solution is fed via 44 to heating zone
12 of the first concentration stage. Heating zone 12 may, for instance, be designed
as a vertical tube heat exchanger, the urea solution to be concentrated being passed
through the tubes. The gas phase obtained in gas-liquid separator 9, a gas mixture
containing ammonia, carbon dioxide and water vapour, is discharged via 43 and passed,
together with the gas mixture containing ammonia, carbon dioxide and water vapour
that is obtained in heat exchanger 10 and discharged from it via 42, to low-pressure
condensation zone 11 and condensed in this zone with an aqueous solution supplied
via 54, for instance process condensate. The carbamate solution obtained in low-pressure
condensation zone 11 is passed via 45 into the shell side of heating zone 12. This
shell side is further fed with the gas mixture containing ammonia, carbon dioxide
and water vapour that is obtained by joining streams 33 and 38, countercurrent to
the urea solution to be concentrated. Supply of the carbamate solution preferably
takes place at a point between the supply of the urea solution to be concentrated
and the supply of the gas mixture to be condensed. Condensation in this way of the
gas mixture by means of the carbamate solution supplied via 45 produces enough heat
to meet the heat requirements of the first concentration stage, in which the urea
solution supplied via 41, which contains 70-75 wt.% urea, is concentrated to a urea
content of 85-95 wt.%.
[0023] The carbamate solution formed in condensation of the gas mixture in the shell side
of the heat exchanger of first concentration stage 12 is discharged via 46, brought
up to synthesis pressure by means of carbamate pump 16, and introduced into scrubbing
zone 5 via 47. The water vapour from the mixture of concentrated urea solution and
water vapour that is discharged from the first concentration stage via 48 is separated
via 49 in water vapour separator 14, while the concentrated urea solution is passed
via 50 to the heating zone of second concentration stage 13. The mixture of virtually
water-free urea melt and water vapour that is formed here is passed via 51 to water-vapour
separator 15, from which via 52 the water vapour is discharged and via 53 the virtually
water-free urea melt. The latter can, for instance be processed to granules or to
a urea-ammonium nitrate solution. The water vapour from the concentration stages is
condensed and the process condensate thus obtained can be treated in part or in its
entirety in the customary manner to remove urea and ammonia and subsequently be discharged.
Example
[0024] Using the process described, urea is prepared according to the embodiment represented
in the figure in a plant with three decomposition stages with a production capacity
of 1000 tonnes a day. The amounts are given in kg an hour. The pressure applied in
the highpressure part of the plant is 139 bar, that in the second decomposition
stage 21.5 bar and that in the last decomposition stage 5 bar.
[0025] High-pressure condensation zone 3 is fed with 23,611 kg NH₃ of 40◊C and 41,647 kg
of a carbamate solution with a temperature of 165◊C, which contains 17,381 kg CO₂,
17,466 kg NH₃ and 6,790 kg H₂O. The temperature in reaction zone 1 is 183◊C and the
NH₃/CO₂ molar ratio 3.2. Stripping zone 2 is fed with 68,202 kg urea synthesis solution,
which solution is stripped with 16,530 kg CO₂ while heat is being supplied. Via 40,
a solution containing 22,541 kg urea, 3,323 kg CO₂, 2,524 kg NH₃ and 10,146 kg H₂O
is discharged from the stripping zone, and via 25 a gas mixture consisting of 25,149
kg CO₂, 19,904 kg NH₃ and 1,145 kg H₂O. Contact zone 6 is fed with 54,092 urea synthesis
solution, and this solution is treated here with 14,025 kg CO₂. From the bottom of
contact zone 6, a solution is discharged that contains 19,222 kg urea, 11,562 kg CO₂,
10,619 kg NH₃ and 8,874 kg H₂O. From the top of this zone, a gas mixture is discharged
that contains 10,949 kg CO₂, 6,407 kg NH₃ and 484 kg H₂O. The pressure of the solution
discharged from contact zone 6 is subsequently reduced to 22 bar. As a result, in
gasliquid separator 7 4,234 kg is obtained of a gas mixture containing 2,715 kg CO₂,
1,375 kg NH₃ and 144 kg H₂O, which is discharged via 33. In addition, 46,043 kg remains
of a liquid phase containing 19,221 kg urea, 8,847 kg CO₂, 9,244 kg NH₃ and 8,730
kg H₂O.
[0026] The gas mixture discharged from stripping zone 2 and contact zone 6 via 27 is in
part condensed in first high-pressure condensation zone 3, yielding 86,169 kg of a
carbamate solution.
The residence time in this zone of the mixture is chosen such that also 17,234 kg
urea is formed in this solution, which in addition contains 24,989 kg CO₂, 31,404
kg NH₃ and 12.543 kg H₂O. A further portion of the non-condensed gas mixture discharged
via 34 is condensed in second high-pressure condensation zone 4, so that synthesis
zone 1 is fed with a solution containing 34,336 kg CO₂, 42,702 kg NH₃, 13,126 kg H₂O
and 17,234 kg urea and with a gas mixture containing 6,506 kg CO₂, 14,831 kg NH₃ and
463 kg H₂O.
[0027] In first high-pressure condensation zone 3 the heat released is discharged by means
of liquid stream 32, resulting in decomposition into NH₃ and CO₂ of carbamate present
in this stream. After the reaction mixture thus obtained has been subjected to a
gas-liquid separation, at a pressure of 21.5 bar and a temperature of 159◊C, via
38 a gas stream is obtained that consists of 7,861 kg CO₂, 6,616 kg NH₃ and 1,546
kg H₂O, and via 39 a solution which, besides 19,126 kg urea, contains 1,056 kg CO₂,
2,684 kg NH₃ and 7,156 kg H₂O. This solution and the solution discharged from stripping
zone 2 are expanded to a pressure of 5 bar and the mixtures obtained are passed into
gas-liquid separator 9. Via 43, a gas mixture consisting of 2,805 kg CO₂, 2,308 kg
NH₃ and 1,081 kg H₂O is passed to condensation zone 11. The solution discharged via
41, which contains 41,667 kg urea, 1,574 kg CO₂, 2,900 kg NH₃ and 16,221 kg H₂O, is
heated to 130◊C in heater 10. This yields, via 42, a gas mixture containing 872 kg
CO₂, 882 kg NH₃ and 520 kg H₂O. For the condensation of the gas mixtures fed to low-
pressure condensation zone 11 via 42 and 43, via 54 5,920 kg process condensate containing
702 kg CO₂ and 2,018 kg NH₃ is supplied. After expansion in expansion valve 20, the
urea solution obtained via 44, which consists of 41,667 kg urea, 702 kg CO₂, 2,018
kg NH₃ and 15,701 kg H₂O and has a temperature of 130◊C, is passed to heating zone
12 of the first concentation stage. The heat required for concentrating is obtained
by condensing the combined gas streams 33 and 38 in the shell side of heating zone
12, use being made of the carbamate solution obtained in the low-pressure stage and
supplied via 45, which contains 4,379 kg CO₂, 5,208 kg NH₃ and 4,802 kg H₂O and has
a temperature of 63◊C. From heating zone 12, a gas-liquid mixture is discharged to
gas-liquid separator 14, from which, at a pressure of 0.38 bar and a temperature of
130◊C, via 50 43,871 kg urea solution in water is obtained, which contains 41,667
kg urea, 2,193 H₂O and traces of NH₃.
[0028] Stripping zone 2 is fed with 20,300 kg steam of 23.5 bar and 221◊C, which corresponds
with 487 kg per tonne of urea produced. In second high-pressure condensation zone
4 17,800 kg low-pressure steam of 5 bar is produced. Of this, 2,600 kg is utilized
in heat exchanger 10 and 2,630 kg in heating zone 13 of the second concentration stage.
The remaining amount is used for maintaining, by means of steam ejectors, the required
vacuum in water vapour separators 14 and 15 in the wastewater purification plant (not
shown).